1. Field of the Invention
The present invention relates to an electrophotographic printer which performs a reliable transfer operation.
2. Description of the Related Art
An electrophotographic printer involves the following sequence of operations. The electrostatic latent image is formed by first charging the surface of a photoconductive drum and then exposing the charged surface to a light pattern. The latent image is then developed with toner, transferred to a print medium such as paper, and then fused to the paper. A tandem type color electrophotographic printer includes a plurality of photoconductive drums spaced apart along the path of a print medium, so that yellow, magenta, cyan, and black images are transferred in registration with each other and fused on the print medium. A tandem type color electrophotographic printer provides higher printing speed than a single drum type color electrophotographic printer where a single drum is subjected to repeated image production processes to print images of different colors. A further advantage of the tandem type color electrophotographic printer is that the conveyer belt can be made substantially flat, and lends itself to printing on a thick print medium.
The conventional electrophotographic printer suffers the following drawbacks.
A transfer electric field is selected so that the entire toner image can be properly transferred to the print medium. The transfer electric field is significantly influenced by characteristics such as impedances of the transfer rollers, print medium, and photoconductive drums. With the tandem type electrophotographic printer, a conveyer belt runs between the transfer roller and the photoconductive drum at each print engine. Thus, a small amount of current flows through the conveyer belt, affecting the transfer electric field. A low impedance at a location where the transfer operation takes place causes a large transfer current to flow and a small change in impedance causes a significant change in transfer electric field. Thus, some measure should be taken in order to increase the impedance at the location where the transfer operation takes place. A technique has been proposed where the surface of the photoconductive drum is exposed to light for neutralizing the charged surface immediately before the transfer operation takes place. However, exposing the entire drum surface to the light causes a developed toner image to loose its adhesion to the drum surface, deteriorating the reproducibility and resolution of the image.